1. Field of the Invention
The invention relates generally to control of communication devices in a wireless communication system, and more particularly, to selecting an operational mode of in a communication system.
2. Description of the Related Art
Communication over a wireless channel, in a wireless communication system, can be accomplished using a variety of techniques that facilitate a large number of users in a limited frequency spectrum. These techniques, commonly referred to as multiple access techniques, include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
In a wireless communication system, a signal may travel several distinct propagation paths as it propagates between base stations and wireless communication devices (WCD). A multipath signal generated by the characteristics of the wireless channel presents a challenge to the communication system. One characteristic of a multipath channel is the time spread introduced in a signal that is transmitted through the channel. For example, if an ideal impulse is transmitted over a multipath channel, the received signal appears as a stream of pulses. Another characteristic of the multipath channel is that each path through the channel may cause a different attenuation factor. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different signal strength than other received pulses. Yet another characteristic of the multipath channel is that each path through the channel may cause a different phase on the signal. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different phase than other received pulses.
In the wireless channel, the multipath is created by reflection of the signal from obstacles in the environment such as, for example, buildings, trees, cars, and people. Accordingly, the wireless channel is generally a time varying multipath channel due to the relative motion of mobile wireless communication devices and structures relative to a wireless network infrastructure and thereby create the multipath. Thus, if an ideal impulse is transmitted between wireless communication devices (WCD) and the wireless network infrastructure over the time varying multipath channel, the received stream of pulses changes in time delay, attenuation, and phase as a function of the time that the ideal impulse is transmitted.
The multipath characteristics of a channel can affect the signal received and result in, among other things, interference between the multipath signal instances. Destructive interference, commonly referred to as fading, is the result of the phasing characteristics of the multipath channel. A fade occurs when multipath vectors add destructively, yielding a received signal that is smaller in amplitude than either individual vector. For example, if a sine wave signal is transmitted through a multipath channel having two paths where the first path has an attenuation factor of X dB, a time delay of δ with a phase shift of Θ radians, and the second path also has an attenuation factor of X dB and a time delay of δ, but with a phase shift of Θ+π radians, no signal will be received at the output of the channel because the two signals, being of equal amplitude and opposite phase, will cancel each other. Thus, fading, such as the extreme conditions in the example, can have a severe negative effect on the performance of a wireless communication system. Conversely, multipath signal instances can interfere constructively if the multipath signals arrive in-phase with each other, resulting in an increase in the received signal strength rather than fading in the received signal strength.
WCDs and transceiver nodes in a wireless network infrastructure often have different modes of operation used to improve the transmission of signals between WCDs and the infrastructure in the presence of signal interference, such as fading. Two such modes are transmit diversity and site diversity. Transmit diversity refers to selecting, or modifying, the way in which signals are transmitted between an infrastructure device, such as a base station, and a WCD. Site diversity refers to selecting a desired base station, from a plurality of base stations, with which a WCD will communicate. Diversity modes can be open-loop or closed-loop. In general, open-loop diversity seeks to avoid statically destructive interference, and closed-loop diversity seeks to obtain constructive interference of signals received at the WCD.
The selection of an optimal operating mode is affected by the signal environment of the wireless channel. For example, if the conditions of the signal transmission path of the wireless channel are stable, and relatively unchanging, then it may be desirable to select a particular diversity mode, such as a closed-loop diversity mode. The closed-loop diversity modes can be selected if the closed-loop can track changes in the wireless channel, and is desirable because closed-loop diversity offers the advantage of constructive interference at a receiver. If the conditions of the signal transmission path of the wireless channel are rapidly changing, such that a closed-loop system cannot track the signal path changes in the channel, then it may be desirable to select a different diversity mode, such as an open-loop diversity mode.
There is therefore a need in the art for techniques to improve the selection of modes of operation of diversity for communication between WCDs and the wireless network infrastructure in a wireless communication system.